The present invention relates to a cross-linkable or cross-linked rubber composition having improved hysteresis properties in the vulcanized state which is usable for constituting a tire tread, to a process for preparation of such a cross-linkable composition, to a tread of this type and to a tire having reduced rolling resistance.
Since fuel economies and the need to preserve the environment have become priorities, it has become desirable to produce mixes having good mechanical properties and as low a hysteresis as possible so that they can be processed in the form of rubber compositions usable for the manufacture of various semi-finished products involved in the constitution of tires, such as, for example, underlayers, sidewalls or treads, and in order to obtain tires having reduced rolling resistance.
To achieve such an objective, numerous solutions have been proposed, consisting in particular of modifying the structure of the diene polymers and copolymers at the end of polymerization by means of functionalizing, coupling or starring agents. The very great majority of these solutions have concentrated on the use of functionalized polymers which are active with respect to carbon black, with the aim of obtaining a good interaction between the polymer thus modified and the carbon black.
By way of illustration of this prior art relating to reinforcing fillers formed of carbon black, mention may for example be made of U.S. Pat. No. 3,135,716, which describes the reaction of living diene polymers at the chain end with a polyfunctional organic coupling agent in order to obtain polymers having improved properties. Mention may also be made of U.S. Pat. No. 3,244,664, which discloses the use of tetra-alkoxysilanes as coupling agents or starring agents for diene polymers.
Silica has been used as reinforcing filler in vulcanizable rubber compositions, in particular those intended to constitute tire treads, for a long time. However, this use has remained very limited, owing to the unsatisfactory level of certain physical properties of such compositions, in particular abrasion resistance.
This is why it has been proposed, in order to overcome these drawbacks, to use functionalized diene polymers instead of the non-functionalized polymers which were used before, and in particular polymers functionalized by alkoxysilane derivatives, such as tetraethoxysilanes. For example, mention may be made of U.S. Pat. No. 5,066,721, which describes a rubber composition comprising a diene polymer functionalized by an alkoxysilane having at least one non-hydrolyzable alkoxyl radical, which makes it possible to eliminate the polymerization solvent by steam stripping.
One disadvantage of these functionalization reactions lies in the coupling reactions which accompany them, which generally makes it necessary to use an excess of alkoxysilane and/or intensive mixing in order to minimize these coupling reactions.
Another drawback of these reactions lies in the later carrying out of the steam stripping operation, which is necessary to eliminate the polymerization solvent.
In fact, generally, experience shows that the functionalized polymers obtained undergo changes in macrostructure during this stripping operation, which results in serious degradation of their properties, unless one is limited to using as functionalizing agent an alkoxysilane belonging to a restricted family, such as that described in the aforementioned document U.S. Pat. No. 5,066,721.
Consequently, it emerges from the above that the use of diene polymers comprising an alkoxysilane function to obtain rubber compositions comprising silica as reinforcing filler is not satisfactory, despite the improved physical properties of these compositions.
This is why research has been carried out on other functionalization reactions, always with a view to obtaining such rubber compositions.
By way of example, mention may be made of French Patent Specification FR-A-2,740,778 in the name of Michelin, which discloses the incorporation, in rubber compositions comprising as reinforcing filler silica in a majority proportion (for example comprising a blend of silica and carbon black), of diene polymers bearing at the chain end a silanol function or a polysiloxane block having a silanol end. For example, a functionalizing agent consisting of a cyclic polysiloxane is used, such as hexamethylcyclotrisiloxane. The functionalized polymers obtained can be separated from the reaction medium resulting in their formation by steam extraction of the solvent, without their macrostructure and, consequently, their physical properties, changing.
Mention may also be made of European Patent Specification EP-A-877 047, which discloses the incorporation of such polymers having a silanol function in rubber compositions comprising as reinforcing filler carbon black having silica fixed to its surface.
It has been possible to establish that these polymers impart rubber properties, in particular hysteresis and reinforcement properties in the vulcanized state, which are improved compared with those of control compositions based on non-functionalized diene polymers, and which are at least analogous to those of compositions based on diene polymers comprising an alkoxysilane function.
Mention may also be made of European Patent Specification EP-A-692 493, which establishes that diene polymers bearing at the chain end alkoxysilane groups and an epoxy group result in improved reinforcement properties and in reduced hysteresis losses at small and large deformations.
One disadvantage of these polymers, which comprise a functional group which is active for coupling to silica or to carbon black surface-modified by silica, is that the improvement in the hysteresis and reinforcement properties which they impart to the rubber compositions incorporating them is generally accompanied by a processing ability of the non-vulcanized mixes which is compromised relative to that of non-functionalized xe2x80x9ccontrolxe2x80x9d polymers.
Of the other functionalization reactions studied, mention may also be made of the functionalization of diene polymers by carboxylic acid functions (COOH) at the chain end, which is conventionally effected starting with an initiator containing an alkali metal.
This functionalization reaction may for example consist, as described in the article xe2x80x9cMacromolecules, vol. 22, p. 85 (1989)xe2x80x9d, of reacting the solution of living polymer with carbon dioxide (CO2) in the solid state, at a temperature of the order of xe2x88x9278xc2x0 C.
Said reaction may also consist, as described in U.S. Pat. No. 4,950,721 and WO-A-91/04993, in first reacting the carbon-metal chain end or ends with a deactivating agent, such as 1,1-diphenylethylene or alternatively xcex1-methylstyrene, then effecting the carbonation in the presence of a polar compound (for example tetrahydrofuran) which is present in the reaction medium in a mass fraction of from 2 to 25%, at a temperature not exceeding 20xc2x0 C. (conventionally of xe2x88x9210xc2x0 C.).
British Patent Specification GB-A-921 803 discloses carrying out the carbonation reaction of the living polymer in a zone of turbulence, for example in a T-shaped tube, into which zone a solution of living polymer and carbon dioxide are introduced.
There are also known from East German Patent Specifications DD-A-139 661 and DD-A-236 537 processes for the carbonation of living polymers which essentially consist of spraying a living polymer, for example a lithiated one, into a turbulent current of carbon dioxide characterized by a very high pressure and flow rate.
Japanese Patent Specifications JP-A-07/138 411 and JP-A-07/138 412 both disclose tire treads obtained by the association with carbon black of an elastomeric matrix comprising in a minority quantity a diene polymer comprising a COOH group at the chain end and having been functionalized by reaction with CO2.
U.S. Pat. No. 3,900,532 discloses the association with silica of an elastomeric matrix comprising in a minority quantity a diene polymer comprising a COOH group at the chain end, with the aim of obtaining a pasty composition which may be, on one hand, processed by injection-moulding and, on the other hand, vulcanized without addition of sulphur or of vulcanization accelerator. To this end, this elastomeric matrix also comprises, in a majority quantity, another diene polymer which is not functionalized.
According to the example of embodiment illustrated in this document, the functionalized diene polymer is a polybutadiene, whereas the other non-functionalized diene polymer is a polychloroprene. Furthermore, the functionalized polybutadiene is present in the rubber composition in a quantity of 50 parts by weight per 100 parts of polychloroprene.
It will be noted that this polybutadiene is characterized by a molecular weight of 6,000 g/mol, this very low weight being linked to the high amount by weight of COOH functions, which is close to 2%.
Japanese Patent Specification JP-A-07/292 159 also discloses a cross-linkable rubber composition which comprises silica and an elastomeric matrix comprising in a very minority quantity a diene polymer comprising a COOH group at the chain end. This elastomeric matrix comprises in a majority proportion another diene polymer which is not functionalized, which is formed of natural rubber or of a copolymer of styrene and butadiene in the examples of embodiment quoted in this document. In these examples, the functionalized polymer is present in the rubber composition in a quantity of 5 parts by weight per 95 parts of non-functionalized polymer.
In the manner of the preceding document, it will be noted that the functionalized polymers which were tested are characterized by molecular weights which are at most equal to 38,000 g/mol, these low weights being linked to the high amounts by weight of COOH functions, which are close to 1%.
The present invention relates to a cross-linkable or cross-linked rubber composition having improved hysteresis properties in the cross-linked state which is useful for constituting a tire tread.
A rubber composition according to the invention is based on: an elastomeric matrix comprising at least one diene elastomer which comprises a carboxylic acid function at one or at each of its two chain ends, and a reinforcing filler comprising a reinforcing inorganic filler, and it is such that said diene elastomer has a molecular weight which is greater than 80,000 g/mol, and in that it is present in a majority quantity in said elastomeric matrix.
The subject of the invention also relates to a process for the preparation of a cross-linkable or cross-linked rubber composition having improved hysteresis properties in the cross-linked state.
The subject of the invention is also tires and, in particular tire treads, comprising a rubber composition according to the invention, to achieve reduced rolling resistance.